1. Field of the Invention
This invention relates to an electrophotographic apparatus that uses liquid toner and particularly relates to a method of determining the percentage of toner solids in the liquid toner while the liquid toner is in the printer.
2. Background of the Art
In electrophotography, an organophotoreceptor in the form of a plate, belt, disk, sheet, or drum having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive element, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas, thereby forming a pattern of charged and uncharged areas. A liquid or dry ink is then deposited in either the charged or uncharged areas to create a toned image on the surface of the photoconductive element. The resulting visible ink image can be fixed to the photoreceptor surface or transferred to a surface of a suitable receiving medium such as sheets of material, including, for example, paper, polymeric materials, metal, metal coated substrates, composites and the like. The imaging process can be repeated many times on the reusable photoconductive element.
In some electrophotographic imaging systems, the latent images are formed and developed on top of one another in a common imaging region of the organophotoreceptor. The latent images can be formed and developed in multiple passes of the photoconductor around a continuous transport path (i.e., a multi-pass system). Alternatively, the latent images can be formed and developed in a single pass of the photoconductor around the continuous transport path by sequential imaging and electrostatic deposit of toner. A single-pass system enables the multi-color images to be assembled at extremely high speeds relative to the multi-pass pass system. At each color development station, liquid color developers are applied to the photoconductor, for example, by electrically biased rotating developer rolls. The color developer may be dry pigment particles or it may be a liquid ink made of small colored pigment particles dispersed in an electrically insulating liquid (i.e., a carrier liquid).
This colored developer is provided for use in the printing apparatus usually through an exchangeable developer cartridge, but occasionally also through xe2x80x9crefillingxe2x80x9d of the initial cartridge. It is, therefore, necessary for the user to be informed of the time such an exchange of cartridges or refill of the cartridge becomes necessary. As a result of this need, many different methods of determining toner cartridge life are known. Some of these methods work best with liquid toner; others are best used with dry toner.
The typical dry developer cartridge contains a sensor that is used to determine when the physical quantity of remaining toner drops below a predetermined level. Frequently this level is determined by applying a bias to one probe, roll, member or other element associated with the cartridge and reading that bias across a predetermined gap with another roll, probe, element or member. When the toner level is high, the current is transmitted easily from one probe to the next. When the toner level is low, air fills the gap and the machine is less able or unable to sense the current from the biased member, signaling to a controller that a xe2x80x9cchange cartridgexe2x80x9d indicator is to be activated. See U.S. Pat. Nos. 5,229,821 and 6,349,184 for examples of that format of toner level indication system. This type of sensor is essentially an on/off sensor, asking if there is toner in the cartridge or not.
For liquid electrophotography, a cartridge change indicator is not nearly so simple as the formats that may be used with dry or powder toner systems. As the toner particles within the liquid toner are transferred to the photoreceptor, fewer and fewer toner particles remain within the liquid. The concentration of toner particles in the toner liquid are not necessarily uniformly constant in the deposited or transferred toner, changing the concentration of toner particles in the liquid, although the level of liquid in the cartridge might remain the same or very nearly the same. Some prior art uses a light beam and window to determine the optical density of the liquid in the developer pod. If the optical density is above a certain value, there are enough toner particles in the developer to keep printing images of useful optical density levels. One problem with this solution is that sometimes the toner particles adhere to the window through which the light beam is directed at the liquid toner, causing the optical density measurement to be falsely high and failing to notify the user that the cartridge does not contain sufficient toner to keep printing. See U.S. Pat. No. 5,530,530 for one example. U.S. Pat. No. 6,337,959 also discloses in its background three additional conventional liquid level measuring apparatuses or methods.
To solve some of the problems associated with determining when it becomes necessary to exchange or refill a liquid toner cartridge, an easier method can be implemented by determining what concentration of toner solids remains in the liquid. This can be determined independently of or in concert with a determination of the absolute amount (weight or volume) of the liquid toner remaining. When that determined percentage falls below a unique threshold, it is time to change the cartridge, add a concentrate to raise the particle level (replenishment) or add toner (replacement). This concentration determination is not the same as measuring what quantity of liquid is in the cartridge, as with dry toner, since the liquid level may vary within a wide range of acceptable limits, while the actual concentration of charged toner particles in the liquid is what is being measured as an important determinant in toner replacement or replenishment.
In the invention, a method for determining the concentration of toner solids present or remaining in any quantity of liquid solvent is described. One embodiment of the invention involves a series of steps. An electrical signal generator is electrically connected to a first electrode. A second electrode, attached or electrically connected to a detecting device, is positioned at a prescribed gap distance (e.g., between 0.005 inches and 0.250 inches) from the first electrode. The two electrodes are submerged in a liquid printing ink (in the practice of the invention in an electrophotographic imaging system, within the toner cartridge), maintaining the prescribed gap distance from one another. The signal generator then transmits an alternating current electrical signal (AC signal) or a direct current signal (DC signal) having a known amplitude to the first electrode. The direct current signal may be pulsed, and the receiving/signaling system may respond to the lack of pulses over a period of time to indicate depleted toner. The second electrode then receives any residual signal that is transmitted or propagates across the prescribed gap distance. The amplitude of the received signal is either detected at an acceptable intensity or determined to be absent or below the acceptable level, and a warning is generating based on whether the signal is received at the acceptable level or not received at an acceptable level (the unacceptable level including no signal received). Additionally, decisions may be made based on the amplitude of the received signal.
In one embodiment of the method, the signal generator""s output is, by way of a non-limiting example, between about 0.1 and 10 MHz, e.g., between 0.5 and 5 MHz, such as at 1 MHz. In a preferred embodiment of the method, the first and second electrical emitting and electrical sensing elements, e.g., the probes are between 0.035 inches and 0.045 inches apart.
There are various possible embodiments and formats of positioning of the detection device element that may be used to send the xe2x80x9cchange inkxe2x80x9d message. In the most basic embodiment, the electrical path is in a simple series connection with the sensing device. When insufficient current is received at the second electrode, the electrical series connection is broken and a light, LED, or other signaling device ceases to relay a message to a receiver or fails to provide a visual signal that the cartridge or ink is still good. In this embodiment, the absence of the signal to an observer is the warning indicator or the absence of the internal light signal may trigger a second imaging feature to engage to provide a visible light signal.
In another embodiment, the AC signal is converted to analog and can be read, interpreted or processed by such devices as meters, processors, and the like. In such an embodiment, the percentage of toner solids remaining in a cartridge may be accurately determined at any given time.
The invention includes a method for determining the percentage of toner solids remaining in liquid printing ink within a liquid printing ink storage device or use cartridge comprises the steps of providing an electrical signal generator on the storage device providing a first electrode electrically connected to the signal generator, the first electrode submerged in the liquid printing ink, providing a second electrode at a predetermined gap distance from the first electrode, the second electrode submerged in the liquid printing ink and connected to an electrical signal detector, transmitting an alternating current signal having an amplitude from the signal generator to the first electrode, detecting the AC signal or its absence with the electrical signal detector, determining the amplitude or absence of the AC signal at the second electrode, and generating a low toner solids warning signal in response to the amplitude being detected at the second electrode at an amplitude below a predetermined minimum level. The second electrode may be, by way of non-limiting examples, between 0.005 inches and 0.250 inches from the first electrode. The method may provide an indicator light that stays on in response to sufficient amplitude, and goes out in response to insufficient amplitude when the alternating or direct current signal is detected and its amplitude determined. The alternating current signal may be picked up at the second electrode by a detecting device which converts the alternating current signal to an analog signal. The resultant analog signal may be sent to a processor for conversion or translation into a warning indication for the user.